Sonic drying of webs

ABSTRACT

A paper dryer is provided with a sonic drying section through which the web is passed and subjected to high intensity noise from grouped noise generators, usually air operated, to dislocate moisture from the web for enhanced drying rates.

United States Patent [is] 3,694,926

Rodwin 8t 81. 1 0d. 3, 1972 [54] SONIC DRYING OF WEBS 3,346,932 10/1967Cheape ..34/i55 X Inventors: Montreal Quebec; Eustace Cosmo Shuffler,3,169,507 2/1965 Rich 16/137 A Lachine, Quebec, both of Canada DominionEngineering Works, Limited, Lachine, Quebec, Canada Filed: June 17, 1970Appi. No.: 46,979

Assignee:

Int. Cl ..F26b 19/00 Field of Search ..34/23, 69, 155, 158, 18

References Cited UNITED STATES PATENTS 4/1956 Fowie .34/155 X PrimaryExaminer-Carroll B. Dority, Jr.

Attorney-Raymond A. Eckersley, Frank L. Neuhauser, Oscar B. Waddeil andJoseph B. F orman [57] ABSTRACT 4Clairm,7DrnwingFigures PAT ENTEB 3SHEET 2 0F 3 FIG. 2

INVENTOR. STEPHEN A. RODWIN EUSTACE C. SHUFFLER PAIENTEDncI 3 m23.694.926

sum 3 or 3 INVENTOR. STEPHEN A. RODWIN EUSTACE C. SHUFFLER BY Z L) SONICDRYING OF WEBS This invention is directed to apparatus for dislocatingmoisture from a wet web using sonic generators.

In the pulp and paper industry a large portion of the cost of makingpaper arises from the need to dewater the pulp stock by the removal ofthe approximately 98 percent by weight water content of the stock.Conventionally, the removal by mechanical means of moisture from the webterminates at the end of the press section. From there on, furtherdrying is usually achieved by more costly thermal drying, in the driersection. The present invention extends the use of mechanical removal ofwater throughout the drier section, with conseq uent economic benefit.

The present invention provides an apparatus for obtaining the mechanicaldislocation of liquid within a web by the application of soniclyvibrating gaseous fluid thereto either alone or in combination with animpingement gas flow for the removal of the dislocated liquid. Whilesuch liquid is usually water, such is not necessarily the case, as otherliquids in furnish are also known.

it has been found that the application of longitudinal sound waves to aweb such as a felt or a paper web at appropriate levels of sound energywill promote transverse dislocation of liquid relative to the web.

In order to transport moisture from the surface or surface of the web,it is necessary to provide a gaseous or vapor flow against a surface orsurfaces of the web. it has been found that by utilizing pressurizedfluid medium such as air initially as the energizing source for a soundgenerator such as a stem jet whistle, and then passing the air from thewhistle against the web, that the fluid can serve the dual function ofwhistle energization and the at least partial wiping and transportationof the released moisture mist away from the surface.

The raising of the temperature of the energizing fluid enhances soundenergization. However, in the matter of transporting dislocated moisturefrom the web partially moist air, such as air issuing from the soundsources, after having picked up web moisture either as finelydistributed mist or in vapor form, is effective as a moisturetransporting agent, while also enhancing the sound field due to itscharacteristic of reducing sound attenuation, relative to a dry airsound field. Web moisture may be carried in mist droplet fonn by the airstream, being dispersed by the sound field into micro droplets asopposed to passage only in the vapor form for conventional heat dryingprocesses, thus enhancing the economics of extraction in liquid form.

It has been further found that the effectiveness of the soundgenerators, such as stem jet whistles may be enhanced by the provisionof suitable sound reflectors, thus improving considerably the efficiencyof sound utilization.

The present invention may be utilized with stationary webs. However, inorder to meet production requirements of rapid dewatering for movingwebs such as in a paper machine, the present invention is utilized inrelation to a moving web, in which high web passage speeds may beachieved. It will be understood that the effect of increased speed mayenhance drying rates due to improved action of the sonic field in theweb boundary. Hence this process is particularly suited for high speedmachines.

A particular advantage afforded by the present invention relates to theadaptability thereof with existing paper drying machinery whereby thedrying rates of the combined plant may be increased, thereby permittingthe speed of the paper making machine to be correspondingly increased.In addition, owing to the general tendency of sonic drying toconcentrate moisture at the web boundaries the efi'ects of othersubsequent surface drying processes are generally enhanced. in addition,the fact, that sonic drying does not result in such a large temperaturedrop as is normally experienced in conventional evaporative drying dueto evaporative cooling, makes the achievement and maintenance ofsuitable web temperatures more economical.

In utilizing a sonic field having sound generators such as stern jetwhistles, it is necessary that the sonic energy input to the paper webshall not exceed the approximate value of 10 watts per square centimeteras chemical decomposition or donolysis may start to occur at this energylevel. Thus the use of a maximum of up to 169 decibels (db) may beregarded as quite safe, in providing a maximum of 8 watts/cm, ascompared with the 10 watts/cm which 170 decibels represents.

[t has been found that the apparent inefiiciency of a free open soundfield is greatly improved when in web drying it is applied as areflective field. Applied sound energy initially provides mechanicalmoisture dislocation, and subsequently a considerable portion of thegenerated sound energy is transformed into heat, hence supplying astaged energy transformation which benefits the overall drying process.Similarly, in providing uncooled compressed air as whistle energizingmedium, the mechanical losses of the air compressor appear as atemperature rise in the whistle air to promote effective compressor andweb drying efficiency while also enhancing whistle operation, ifeffectively utilized.

From psychometric charts, it is known that the requirement of energy formoisture removal from a web is diminished if the mean temperature of theair dyring the web is maintained as high as possible owing to favorablechanges in it tension and viscosity which result. lr has been found thatthe action of a sonic field in an air streal also favorably changesboundary surface tension and heat transfer coefficients to enhancedrying, while also suitably conditioning the inside of the porous webstructure. Thus higher temperature air has been found highly beneficialin promoting heat and mass transfer to improve drying rates.

The effective use of sound generators such as stern jet whistles indrying a moving paper web require the capability of exercising controlover the web drying rate, both generally and locally. One characteristicof a sonic field is the advisability to locate the plane of the paperweb substantially perpendicular to the sound wave generally at distancesrepresenting multiples of half the wave length from the sound source. Ingeneral, a change in air flow conditions at the whistle inlet leads to achange in frequency with a consequent change in wave length. Forpermanently set whistles this would probably reduce resonation, so thatresonation air flow ranges have to be established and maintained. Forthis reason, it is not practical to permit substantial changes to theair flow rates for a particular whistle arrangement, as the deviation ofsonic conditions from the design point would not permit consistency indrying. In order to provide control of the rate of moisture removal withother than acoustic means, it is necessary to effect control over theadditional moisture removal media, namely by controlling the wiping orimpingement air flow within a predetermined range of mass flow, velocityand psychometric conditions.

Wiping or impingement air may be supplied in a variety of ways to affordcontrol of drying rates. When air is used for actuating the sonicgenerator, it is generally possible and desirable to utilize resonatedexhaust process air, with or without secondary air of low moisturecontent, by passing the air as a wiping fluid across the web surface.Low velocity air wiping may be achieved utilizing pressure build up inthe sound field. The control of air escape from the sonic zone bysuitable throttle means is then readily provided, to effect the desiredcontrol over moisture removal by variation of the wiping air flow rate.Alternative to the foregoing provision or additionally thereto, theremay be provided air pressurizing means such as a fan or blower, for thedirect supply of secondary air in bypass relation to the whistle. Suchby-pass secondary air may be passed through the resonating fieldproduced by the whistles. Local control by adjustment of wiping air flowpermits the eradication of wet streaks in the web. Such bypass secondaryair may be fresh air or recirculation air, or a mixture of fresh andrecirculation air to provide desired psychometric conditions.

The operation of the sound generation means is enhanced by the provisionof sound reflector means to direct the sound energy against the web andby reflection to return a considerable portion of energy to the sourcein re-energizing relation therewith. In addition, at the boundaries ofthe sonic drying zone, the provision of auxiliary sound reflectors tominimize the passage of sound energy from the zone enhances the soundintensity within the zone while reducing the provision of sound proofingnecessary to reduce outside noise to practical working levels. Theprovision of perforated reflectors adjacent the web entry and exitlocations permits the passage of air therepast in web wiping relation,while enhancing the performance of the sound chamber. The sonic energyabsorption losses in reflectors is effectively returned to the processin the form of heat energy.

While theoretical considerations of the application of compressed airagainst a moving web, applied initially as sound energy in energizing awhistle might lead to the conclusion that efficient utilization willproduce rather low air temperature, it has been found that theappearance of sonic losses as heat energy, from the effects previouslydiscussed, maintain the air temperature at a preset level, of forinstance 200 F. Thus high recovery of sound energy as useful heatenhances the overall process efficiency. While the described embodimentsare directed to single and double sided impingement against a planarweb, the utilization of the present invention with a dryer cylinder isalso contemplated.

In obtaining the full benefits of sonic web drying, removal of moisturedisplaced to air adjacent the web surface is particularly important, soas to avoid rewetting of the web. One very effective means for boundarymoisture removal is the use of impingement air jets, as a form of airwiper.

The particular advantages obtained in utilizing sonic drying againstboth sides of a web is the improvement in drying rates and the balancingor manipulation of transverse forces acting upon the web. In the case ofa symmetrical, balanced arrangement, the use of a support web or wiremay on occasion be dispensed with.

In addition to the effectiveness of the foregoing disclosed dryingprocess, it will be understood that particular advantages accrue incombining a short sonic dryer section with an existing conventionaldryer, generally locating the sonic section at the inlet or wetter endin order to fully utilize the characteristic higher dislocation ratesachieved with the wetter web. The improved overall performance of aso-modified dryer section permits up-rating of the speed of the completepaper machine in cases where the primary limitation to such up-ratinghas been dryer performance.

The subject process includes the supply of compressed air to the soundgenerators. The raising of the temperature of the supply air enhancesboth drying and whistle efficiency. To produce higher temperature supplyair, the choice exists between heating of the already hot compressoroutput air by independent heating means or more effectively utilizingthe compressor to produce the required exhaust temperature, by raisingcompressor inlet temperature slightly above ambient, providing intakeair in the range of F to F at the compressor inlet. For most of theconsidered embodiments, the latter course is more economical in heatenergy requirement, while capita cost is minimized.

Certain embodiments of the present invention are described, referencebeing had to the accompanying drawings wherein:

FIG. 1 shows two perspective sectional arrangements of embodiments ofthe subject invention showing the use of transverse trough-like soundgenerators, and individual sound reflectors, respectively;

FIG. 2 is a side sectional view showing a first embodiment having anunsupported web and using only primary or whistle air as the moisturetransport medium;

FIG. 3 is a side sectional view showing a further embodiment having aresonant chamber receiving secondary air;

FIG. 4 is a further embodiment similar to that of FIG. 1, with secondaryair circulation provision;

FIG. 5 is a view similar to FIG. 3 of a further embodiment;

FIG 6 is a sectional detail of an edge seal suitable for use with thesound chambers illustrated in FIGS. 2 5', and

FIG. 7 shows characteristic drying rate curves, and appears with FIG. 1.

Referring to FIG. I, this illustrates the form of two alternative soundgenerator arrangements, the first showing trough-shaped reflectors 18 inseries arrangement within a sound hood, each trough reflector having aplurality of stem jet whistles herein, while in the second arrangementis shown a plurality of whistles, each with an individual reflector 18'and mounted in staggered relation.

Referring to FIG. 2, this shows a vertical unsupported web run whereinthe drier section 10 has a first sonic chamber 12, and an opposing sonicchamber 14, each chamber having a cap or cover 16 comprisingreverberation plates. Each chamber 12, 14 is provided with a pluralityof sound generators 18 to propagate longitudinal sound waves against theweb 20. The chamber may be in horizontal or vertical arrangement,depending generally on the provision of web support netting and thesymetry of incident energy, in relation to the grade of paper.

Each sound generator 18 comprises a trough-like reflector having aplurality of spaced whistles, such as a stern jet whistle 26 providedwith air from a compressor by supply conduit means 24, and mountedwithin sound reflecting means 28, 30 to concentrate both thelongitudinal sound emission and sound reflection at the surface of theweb 20, the distance A" representing very nearly a multiple of thewhistle half wave length at desired operating conditions. The individualwhistles 26 are spaced laterally in the reflecting means 28, 30 atsubstantially uniform distances having multiples of half wave lengthapart to minimize mutual interference. Intervening baffles may be used.

The reverberating cap or cover 16 defining a sonic chest also is locatedat a selected distance B from the web surface, equal to approximately amultiple of the whistle half wave length. In supplement to thereflecting surfaces 28, 30 and 16 the reflector plates 23 adjacent theweb path also serve to retain sound energy within the section 10. Thealternative provision of individual whistles having circular individualreflectors of similar diametral section to that of the trough reflectorsis also contemplated.

The sound generators 18 including supporting side walls 25 havingapertures 32 extending therethrough and connecting with vents 22 in thecover 16 to permit the passage of air from the whistles in fluid wipingrelation over the surface of web 20, for exhaust to the hooded zoneabove the section 10, by way of the apertures 22, shown as having soundrelfecting bafl'le means 11 therein to maintain a low sound level in thevicinity of the drier section.

In addition to the provision of trough end walls, the provision ofintermediate separating walls between at least some of the adjacentwhistles is contemplated.

In the embodiments of FIGS. 3, 4 and 5 the sound reflectors 31, 41 and51 also are illustrated as extending for the width of the respectivedrier section 10. The alternative use of individual circular reflectorsshown in FIG. 1 shows a mutual staggered arrangement to providedistribution of sound field across the web.

The arrangement of FIG. 4 shows air circulator 45, having an enclosuremounted on the cover 16 of the sonic chest with an air circulating fan41 mounted therein. The apertures 37, 44 and 39 provide access forwiping air recirculating within the sonic chest of the drier section 10,while the dampers 43 and 47 operating in conjunction with apertures 51and 53 provide blending elation with hood air from outside the sonicchest for a desired combination of hood air and recirculation air inaccordance with desired psychometric conditions. The setting of thedampers 43 and 47 is controlled by the levers 49 and 50.

FIG. 5 shows a horizontal arrangement wherein a support wire mesh 521carries the web 520 in supported relation between the opposed soundchests 510. Each chest 510 contains one or more troughlike reflectors518, each reflector containing a plurality of stem whistles 526 inmutually spaced relation. The cross-sectional profile of the roughreflectors 518 differs from the parabolic form of FIGS. 1 and 4. Primaryair from the whistles 526, together with secondary air passing downwardthrough apertures 539, serves to scour the boundary layer from ofl theweb 520. The secondary air is heated by heaters 550, illustrated asbeing of the electrical resistance type. Fan means 541 providescirculation of the secondary air, while the adjustable dampers 545control the inlet access of hood air and the adjustable dampers 547control the portion of air passing from the fans 541 in recirculatingrelation over the surface of web 520 by regulating the portion of mixedprimary and secondary air escaping to the hood. The maintenance ofadequate ventilation is important in prevention of web rewetting bymaintaining suitable psychometric conditions. The sound reflectors orbaffles 560 reduce the escape of noise upwards into the hood space.

In the embodiment of FIG. 6 the wiping of the moist boundary layercontaining dislocated moisture in mistlike or droplet form isfacilitated by the provision of mechanical wiping members 610 servingboth to assist in removal of moist air, to maintain the sound levelwithin the hood by reflection of incident sound waves back into thespace beneath the whistles, and to stabilize the running of the web. Themembers 610 are adjustably mounted to provide adjustable spacing fromthe web surface, with the off-coming gap on the downstream side of asound generator being greater than the on-coming or upstream gap. Theuse of sawtoothed or levelled surface wipers is contemplated.

In the FIG. 3 embodiment a reverberating chamber 303 is provided. Inaddition to incoming whistle air from high pressure ducts 302 servingthe whistles 301 shown mounted in mutual opposed staggered relationconsidered in the cross machine direction, secondary air is recirculatedby fans 341 while heating means 350 impart additional heat to thesecondary air. The sound and heat insulated cap structures 310 areprovided with sound external hood 311 therebetween to minimize noiseloss to the external hood space. Dampers 308 adjustably mounted at therespective cap edges permit control of secondary air recirculationthough the fan means 341. Non-resonating incoming hood air or fresh airis admitted by way of inlets 339, while outward passage of moist air tothe hood space takes place through the outlets 312. The edge soundlosses are minimized by provision of sound reflector plates 323. Thestaggered opposed whistles 301 are mounted in a reverberating chamber303 through which flows the incoming secondary air as circulated by thefans 341. Control of psychometric conditions at the web surface by useof controlled secondary recirculation and makeup air and therecirculation by the fans 341 permits optimized drying. The provision ofadditional stem jet whistles in the reverberating chamber of at leastone of the hoods 310 in excess of the usual requirement permitsselective additional reverberating energization for wet streak control.

Referring to FIG. 7, which shows the variation of drying rate with webdampness, having tree characteristic Curves representing three differentweb speeds, it will be seen that, with the exception of the range 25percent to 35 percent wetness (wet base), the characteristic drying ratecurve has a positive gradient, so that in the majority of the wetnessrange there is a tendency to eliminate wet streaking.

A further and unobvious advantage afforded by the use of sonic drying isthat, for a side range of web wetness the effective drying rateincreases with the wetness of the web. Thus, for the range so affected,there is inherent tendency to control wet streaks.

What I claim as new and desire to secure by Patent patent of the UnitedStates is:

1. Web drying apparatus having a passage for an endless web therethroughfor controlled diminution of liquid within the surface of an elongatedpermeable web, having web positioning means to laterally locate a saidweb relative to the longitudinal edges thereof on passage through saidapparatus, a plurality of sound generating whistle means on one side ofa path of travel of said web in mutual spaced relation arranged toprovide a sound field having a plurality of sequential high intensityzones, spaced along the web path of travel, and to propagatelongitudinal sound waves in a direction substantially normal to the web,first gaseous fluid supply means connected to said sound generatingmeans in energizing relation therewith to generate in operation saidlongitudinal sound waves, second gaseous fluid supply means havingoutlets intermediately of at least some of said zones to provide awiping gaseous flow in a direction substantially normal to the plane ofthe web, plural housing means extending transversely of said web pathrespectively located adjacent at least some of said high intensity soundzones to provide a plurality of gaseous fluid return flow pathsextending away from said web path, to receive said wiping gaseous flowhaving liquid and vapor entrained therewith, and recirculation means torecirculate a selected portion of return gaseous flow to the web, toprovide control of web drying conditions.

2. Apparatus as set forth in claim 1 including sound hood means tointensify said sound field at the surface of a said web.

3. Apparatus as claimed in claim 2 wherein said recirculation meansincludes fan means for returning said selected portion in downflowingbypass relation with said whistle means; and controllable air admissionmeans to admit additional air for mixing with said recirculation air, incontrolling the psychrometric condition of said wiping fluid.

4. Apparatus as claimed in claim 2 including air heating means toprovide temperature control of gaseous fluid within said apparatus.

1. Web drying apparatus having a passage for an endless web therethroughfor controlled diminution of liquid within the surface of an elongatedpermeable web, having web positioning means to laterally locate a saidweb relative to the longitudinal edges thereof on passage through saidapparatus, a plurality of sound generating whistle means on one side ofa path of travel of said web in mutual spaced relation arranged toprovide a sound field having a plurality of sequential high intensityzones, spaced along the web path of travel, and to propagatelongitudinal sound waves in a direction substantially normal to the web,first gaseous fluid supply means connected to said sound generatingmeans in energizing relation therewith to generate in operation saidlongitudinal sound waves, second gaseous fluid supply means havingoutlets intermediately of at least some of said zones to provide awiping gaseous flow in a direction substantially normal to the plane ofthe web, plural housing means extending transversely of said web pathrespectively located adjacent at least some of said high intensity soundzones to provide a plurality of gaseous fluid return flow pathsextending away from said web path, to receive said wiping gaseous flowhaving liquid and vapor entrained therewith, and recirculation means torecirculate a selected portion of return gaseous flow to the web, toprovide control of web drying conditions.
 2. Apparatus as set forth inclaim 1 including sound hood means to intensify said sound field at thesurface of a said web.
 3. Apparatus as claimed in claim 2 wherein saidrecirculation means includes fan means for returning said selectedportion in downflowing bypass relation with said whistle means; andcontrollable air admission means to admit additional air for mixing withsaid recirculation air, in controlling the psychrometric condition ofsaid wiping fluid.
 4. Apparatus as claimed in claim 2 including airheating means to provide temperature control of gaseous fluid withinsaid apparatus.